Coxiella burnetii, the etiological agent of "Q-fever", is a category-B bioterrorism agent that is highly infective to both humans and livestock. Human infection arises primarily from aerosol transmission and C. burnetii can withstand desiccation and remain infectious in contaminated soils for several years. Ease of dissemination via aerosol, environmental persistence, and high infectivity (ID50=1) make C. burnetii a serious threat for military personnel and civilians. This agent has already been weaponized and mass-produced under various biological warfare programs. Consequently, there is an urgent need to develop a vaccine that is effective and can be safely used for targeted or mass vaccination following a presumed or confirmed outbreak. Currently, there is no vaccine that meets these criteria-we propose a strategy to identify defined C. burnetti immunogens that meet these criteria. We propose to identify and test defined C. burnetti immunogens using a novel proteomic approach combined with screening for the required cell-mediated response, an approach that takes advantage of the recent completion of the Nine Mile strain genomic sequence. Intracellular killing of C. burnetti is mediated via IFN-gamma and is associated with strong stimulation of nitric oxide production. Consequently, vaccine induced immunity is dependent upon priming and expansion of CD4+ T lymphocytes that can recognize infected macrophages via MHC class II presentation of C. burnetti antigens and secrete high levels of IFN-gamma. In this project we will test the following hypotheses: i) that defined C. burnetii proteins, identified by ability to stimulate a CD4+ T cell response, will induce protection against virulent challenge equivalent to that induced by the whole-cell vaccine; and ii) that immunization using these defined, immunostimulatory C. burnetti proteins will significantly reduce the incidence and severity of post-vaccination adverse reactions. These hypotheses will be tested in three specific aims: 1] Identification of novel T cell immunogens in protective native C. burnetti whole cells; 2] Conservation of T cell immunogens among C. burnetti strains; and 3] Immunization with C. burnetii antigens and determination of protective immunity against homologous and heterologous strain challenge.